While I disagree with the full premise of the OP; there is definitely something to the idea of Buffalo being far better with McDavid.
Tim Murray built the Sabres for McDavid, not Eichel. Evander Kane being the easiest proof of that since watching games from Eichel's time with Kane shows two players with very similar positioning approaches which tended to put them into weird situations where both were in the same part of the zone. Additionally Kane would tend to just shoot from distance for no discernable reason . The biggest part however was off the ice where Kane had influence with Eichel and Reinhart through Eichel for that manner. Kane's personality meshed well with Eichel's Boston boy upbringing whereas McDavid's good ole Canadian boy persona would of gravitated to ROR or even Gionta.
If I remember correctly, Eichel was rumored to stay in college if drafted by the Coyotes. Whether he would of carried through on his threat is another story altogether.